Code
P0323
HUMMER
P — Powertrain
Ignition/Distributor Engine Speed Input Circuit Intermittent
AI status
Completed
Completed
100%
Causes
- Loose, corroded or damaged connector at the crank/distributor/ignition speed sensor
- Broken, chafed or shorted wiring in the sensor/PCM harness
- Faulty engine speed sensor (crankshaft or distributor pickup or Hall sensor)
- Poor or missing ground or reference voltage to the sensor
- Intermittent internal failure of the ignition module or PCM
- Physical damage or foreign debris at the sensor tone wheel/reluctor/distributor teeth
Symptoms
- Malfunction Indicator Lamp (MIL/CEL) illuminated
- Intermittent stalling, hard start or no-start conditions
- Engine misfire or rough idle that comes and goes
- Erratic or fluctuating RPM/engine speed reading on scan tool
- Loss of ignition timing reference or poor driveability
What to check
- Read and record freeze-frame and all stored/pending codes with a scan tool
- Inspect sensor connector and wiring for corrosion, bent pins, damage or moisture
- Perform wiggle test on wiring and connectors while monitoring engine speed signal with scan tool or oscilloscope
- Check for good battery voltage and solid ground(s) at sensor and PCM
- Compare engine RPM reported by scan tool to actual engine speed and tachometer
- Check related sensors/codes (cam/crank correlation)
Signal parameters
- Typical Hall/optical sensor: 0–5 V square wave output, clean transitions between low and high; frequency increases with engine RPM
- Typical magnetic (inductive) pickup: AC waveform whose amplitude increases with RPM (may not be a steady 0–5 V); waveform should be regular and proportional to speed
- Pulse pattern depends on engine/distributor design — waveform should be consistent without dropouts or noise; consult vehicle service manual for exact pinout and expected amplitudes
Diagnostic algorithm
- Verify DTC and any related codes, then clear codes and attempt to reproduce the fault
- Visually inspect sensor, reluctor/reluctor ring or distributor cap/rotor for damage, debris or oil contamination
- Inspect connector and wiring for corrosion, loose pins, broken insulation or rubbing through; repair any damaged wiring or connectors
- Backprobe sensor signal and reference/ground with a digital multimeter or oscilloscope while cranking and running; look for clean, stable pulses
- If signal is erratic during wiggle tests, isolate and repair wiring harness or connectors at the failing point
- Check sensor air gap/position and mechanical alignment per service manual; correct if out of specification
- If wiring and connector are good and signal is absent or inconsistent, replace the engine speed sensor (crank/distributor pick-up) and retest
- If new sensor and wiring checks do not restore a stable signal, verify PCM input circuitry and grounds; consider PCM testing/reprogramming as a last step
- After repairs, clear codes and perform a road/test cycle to confirm issue does not return
Likely causes
- Corroded/loose connector at the speed sensor
- Broken/chafed wiring where harness flexes (engine/vibration areas)
- Failing/cracked speed sensor
- Poor sensor air gap or damaged reluctor/trigger tone
Fault status
Status
P0323 — Ignition/Distributor Engine Speed Input Circuit Intermittent: intermittent or erratic engine speed reference signal detected.
Repair difficulty: Medium
Diagnostic time: 1.0 - 3.0 hours
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Repair manuals
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138
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